1) Field of the Invention
The field of this invention relates to testing equipment and more particularly, to the method of making a diffusion cell which is utilized to determine the transfer of a substance through a membrane into a liquid placed within the diffusion cell.
2) Description of the Prior Art
Percutaneous absorption test cells are used in the study of kinetics of the partition coefficients defining the passage and equilibrium states of components into and through a barrier separating two dissimilar substances. A typical test cell is known as the Franz cell. The Franz cell is in the form of a container with an upper compartment separated from a lower compartment by a porous membrane comprising a barrier. A clamping arrangement is located between the upper compartment and the lower compartment with the barrier in the form of a thin sheet of material to be placed and clamped tightly in a stretched configuration by the clamping arrangement.
The lower compartment of the diffusion cell or Franz cell is completely filled with a receptor media in contact with the barrier. In the upper compartment there is placed a donor substance which may comprise a solid, semi-solid, gas or liquid. The receptor media normally comprises water, a buffer solution or saline solution, but may consist of any polar or non-polar liquid, semi-liquid, or gas with or without surfactants added. Connecting the lower compartment is a sample port and it is through this sample port that aliquots are to be removed. An aliquot is defined as an exact subvolume of the overall volume of the receptor media.
While generally useful in the whole field of physical chemistry, diffusion cells or Franz cells have become particularly useful in the health care field. Levels of epidermal exposure to pesticides, chemicals, ointments and cosmetics are important in the field of environmental science. The rate of release and absorption of ingredients for medicinal skin patches is essential to determine the size, dosage amount and characteristics of the patch.
There has been designed automatic testing equipment in which a given machine will use a plurality of these diffusion cells. The machine will automatically affect the removing of the aliquots and the replacing of the lost fluid within each diffusion cell. The removing of the aliquots is accomplished over time with typical removing occurring sequentially each minute. Each aliquot is then tested to determine the amount of the donor substance that has actually penetrated and becomes dispersed within the receptor media.
It is exceedingly important for accurate results that the volumetric size of the lower compartment of the diffusion cells be constant. Since these diffusion cells are normally constructed individually and are not mass produced, in the past it has been difficult to achieve a high degree of accuracy from one diffusion cell to another.